1. Field of the Invention
The present invention relates to an image processing technique for processing digital images and, more particularly, to a technique for separating a screened image in the form of binary data into a first region from which a gradation area is to be derived and a second region from which a monotone area is to be derived, to selectively convert the first region into a multi-level gradation image.
2. Description of the Background Art
In a printing company and the like, there are cases where output using screened image data (with a resolution of, e.g., 2400 dpi) involves a need to change the screen ruling thereof or to readjust the dot gain thereof, depending on printing machines to be used. There are other cases where a change in color or tone of part of a picture image is requested for output reusing the screened image data stocked or stored after use. In these cases, it is possible to create desired screened image data again by performing a RIP process on image data (layout data) described, e.g., in PDF (Portable Document Format) from which the screened image data is generated. This technique, however, is disadvantageous in increased costs and increased processing time.
To overcome the disadvantage, another technique has been conventionally attempted which includes the steps of performing a descreening process for re-creating layout data directly from screened image data without the need to go back to the RIP process, making a desired correction, and then performing a screening process for generating screened image data again.
Specifically, the descreening process refers to the process of re-creating layout data with a resolution of about 300 to 400 dpi and having multi-level gradation mainly from a picture image portion of screened image data which is binary image data with a resolution (output resolution) of about 2400 to 4000 dpi.
In general, a picture image (including photographs and patterns) having multi-level gradation and a monotone character/line having no gradation are mixed on printed material. A screened image accordingly includes two types of regions: a first region from which a gradation area on the printed material is to be derived, and a second region from which a monotone area on the printed material is to be derived. In the first region, halftone dots are formed after the RIP process, i.e., the screening process. On the other hand, it is essentially unnecessary to perform the descreening process on the second region because the layout of binary data with a resolution as high as the output resolution is done in the step of generating the layout data. Thus, the screened image including both of the first and second regions, for example, in which a character/line is drawn on a picture is constructed to contain the first region in which halftone dots are arranged with predetermined regularity, and the character/line overlaid on the first region independently of the arrangement. It is therefore desirable that the descreening process is performed only on the first region after the first region and the second region in the screened image data are precisely separated from each other.
The technique of precisely separating the first region and the second region is also required, for example, when it is desired to extract only character information from the screened image.
There are already known such techniques for separating the first region and the second region in the screened image data. For example, Japanese Patent Application Laid-Open No. 2002-252756 discloses the technique of judging whether or not a recorder grid of interest belongs to the first region, depending on whether or not the sum of the differences between the gradation level of the recorder grid of interest and the gradation levels of four recorder grids adjacent thereto satisfies a predetermined reference value for judgment.
Simple conversion of the resolution is accomplished by uniformly performing a weighted averaging process on the screened image. This, however, presents the problem of degraded image quality, such as a generally blurred resultant image as compared with the original layout data or change in color. To solve the problem, studies have been conducted on the technique of performing the descreening process more accurately by giving consideration to the shapes of individual halftone dots. This technique is disclosed, for example, in Japanese Patent Application Laid-Open No. 2000-224415.
The high-accuracy descreening process is the process of precisely separating only the first region of interest from other regions to precisely reproduce a multi-level gradation image yet to be RIP-processed from the separated first region. Accomplishment of the process allows the re-creation of screened image data subjected to desired correction and the like without the degradation of image quality.
The use of the method of separating the first region and the second region from each other which is disclosed, for example, in Japanese Patent Application Laid-Open No. 2002-252756 will succeed in somewhat rough separation. This method, however, finds difficulty in separating a character/line overlaid on a picture image from the picture image precisely at the output resolution level (about 2400 dpi or higher). An example of this method is shown in FIG. 15. Referring to FIG. 15, the second region CR2 is shown as overlaid on the first region SR1. Conventionally, the second region CR2 might become greater in area by the amount of a region BR1 or smaller by the amount of a region BR2 with respect to a true boundary in a boundary region BR. This causes the layout data obtained by the separation process and the subsequent descreening process to present the problems that the character/line overlaid on the first region SR1 is unsharp or unnatural and that different separation processes are performed on the screened image data for respective CMYK plates.
The method of descreening is also disclosed, for example, in Japanese Patent Application Laid-Open No. 2000-224415. Although performing the process based on the shapes of individual halftone dots, this method performs the descreening process without consideration of how pixels (referred to hereinafter as picture-originated (derived) pixels) constituting the first region are arranged in the layout data yet to be binarized in the RIP process. This might cause the creation of picture-originated pixels differing in arrangement from those in the original layout data, depending on the result of the descreening process.